Sulphonic acids of aromatic diisocyanates and polyisocyanates are known. They are readily obtained by reacting the corresponding aromatic diisocyanates or polyisocyanates with sulphonating agents, such as sulphur trioxide, adducts of sulphur trioxide, oleum, chlorosulphonic acid or sulphuric acid (see, for example German Offenlegungsschriften Nos. 2,227,111 and 2,357,615, and U.S. Pat. No. 3,826,769). It is possible to obtain solid, resin-like or powder-form sulphonation products or solutions of the sulphonated isocyanates in unreacted starting isocyanate, depending upon the isocyanate used and upon the degree of sulphonation.
Although no difficulties are involved in handling liquid sulphonated polyisocyanates, considerable problems arise in the production, storage and use of solid powder-form isocyanatosulphonic acids. These products are obtained in such finely divided form during their production that they are difficult to separate from the liquid reaction medium and to purify. The dried products give off dust during packaging and re-packaging. More importantly, however, dry powders produced in this way are not stable in storage. During storage, the melting or decomposition point increases and the products become increasingly insoluble in organic solvents and in the polyesters, polyethers and polyols normally used in the production of polyurethanes.
Although the problem of adequate stability in storage is generally prevalent in the case of polyisocyanates and is therefore known in principle to those skilled in the art, the deterioration in quality, after production in the case of solid powder-form isocyanatosulphonic acids occurs so quickly (for example after only a few days) that the production of technically useful polyaddition products is made extremely difficult, if not impossible.
On the other hand, there is a commercial need to use polyisocyanatosulphonic acids in addition to or instead of the conventional diisocyanates and polyisocyanates in polyaddition chemistry because these isocyanates are excellent starting products for the production of hydrophilic, especially water-dispersible polyurethanes. In addition, they appear particularly favorable from the physiological and industrial hygiene point of view because they have substantially no vapor pressure and, upon degradation, form water-soluble aminosulphonic acids.
Hitherto, polyurethanes based on sulphonated tolylene diisocyanate have been produced either by sulphonating the prepolymer produced from the pure diisocyanate rather than the diisocyanate itself (U.S. Pat. No. 3,826,769) or by producing the isocyanatosulphonic acid only shortly before further processing into the polyurethane (U.S. Pat. No. 3,826,769). The disadvantage of the first method is that there are limitations on the use of the sulphonating agent because sulphur trioxide, for example, can decompose the polyether prepolymers. If sulphuric acid is used for sulphonation, simultaneous chain-extension to form urea groups is unavoidable. In addition, it is only possible by this method to sulphonate completely or partly free isocyanates, but not products in the form of urethanes. In the case of an NCO-prepolymer, therefore, it is only the terminal isocyanate units which are sulphonated. The second method is generally not performed on a commercial scale because the manufacturer of a polyurethane cannot be expected to carry out an isocyanate sulphonation process beforehand.
It has also been proposed (U.S. Pat. No. 3,826,769) to dissolve the isocyanatosulphonic acids immediately after the production thereof in an organic solvent, for example acetone, and to use them in the form of a solution. This method is also unsuitable for operation on a commercial scale because, for example, a solution of sulphonated tolylene diisocyanate in acetone is only stable for at most a few hours, after which clouding and sedimentation quickly occur.
Accordingly, it is not surprising that solid isocyanatosulphonic acids have hitherto not been adopted for use on a commercial scale.
Accordingly, there was a need to produce and stabilize isocyanatosulphonic acids in such a way that they may be readily stored and used, retaining their solubility in organic media even after prolonged storage.
In addition, there is a commercial need for a solvent for aromatic isocyanatosulphonic acids which is capable of taking up as large as possible a quantity of the isocyanatosulphonic acids and enables these substances to be further reacted in homogeneous phase.